1. Field of the Invention
This invention relates to radiation detection. More particularly, the invention relates to a method and apparatus for passive detection of fissile material with some particular applications in homeland security. Even more particularly, the invention relates to portal monitors for detecting radiation from cargo.
2. Description of the Related Art
The limited inventory and minute natural abundance of 3He gas on Earth necessitate the adoption of new technologies for the detection of neutrons, especially in homeland security applications, where large volume deployments are being considered that would exhaust the entire world supply.
The only practical source of 3He on Earth is through production of the intermediary radioactive tritium (3H) gas. Tritium decays to 3He at a rate of 5.5% per year. Tritium was produced over the time frame from 1955 to 1988 for use as a critical ingredient of nuclear weapons. Production ceased in the US in 1988 and likely will not resume, as there is currently an adequate supply to sustain the diminishing nuclear weapons inventory. The current worldwide production of 3He is estimated at 8 kiloliters per year.
US government plans to equip major seaports with large area neutron detectors, in an effort to intercept the smuggling of nuclear materials, have precipitated a critical shortage of 3He gas. It is estimated that the annual demand of 3He for US security applications alone is 22 kiloliters, more than the worldwide supply. This is strongly limiting the prospects of neutron science, safeguards, and other applications that rely heavily on 3He-based detectors. Clearly, alternate neutron detection technologies that can support large sensitive areas, have low gamma sensitivity, and low cost must be developed.
The background to the present invention and related art is best understood by reference to Applicant's own prior work, including in particularly, U.S. Pat. No. 7,002,159 B2 the '159) entitled “Boron Coated Straw Neutron Detector” which issued Feb. 21, 2006. The '159 is hereby incorporated by reference in its entirety, for all purposes, including, but not limited to, supplying background and enabling those skilled in the an to understand, make and use in Applicant's present invention.
Applicant's other issued patents and pending applications may also be relevant, including: (1) U.S. Pat. No. 7,002,159 B2 (the '159) entitled. “Boron Coated Straw Neutron Detector”; (2) U.S. Pat. No. 8,330,116 entitled “Long Range Neutron-Gamma Point Source Detection and Imaging Using Rotating Detector”; (3) U.S. patent application Ser. No. 13/106,818 entitled “Neutron Detectors for Active Interrogation”; and (4) U.S. patent application Ser. No. 13/106,785 entitled “Sealed Boron Coated Straw Detectors” Each of these listed patents and applications are hereby incorporated by reference in their entirety for all purposes, including but not limited to as background material and as description supporting certain embodiments of the claimed invention.